The intervertebral discs form cartilaginous joints between the end plates of vertebrae to provide shock absorption. The discs include two main regions: the nucleus pulposus, which is an inner, soft and highly hydrated structure, and the annulus fibrosus, which is a strong structure including lamellae (concentric sheets of collagen fibers), which surrounds the nucleus. The three major constituents of the discs are water, fibrillar collagens, and aggrecan. The proportion of these components varies across the disc, with the nucleus having a higher concentration of aggrecan and water and a lower collagen content than other regions of the disc. The loss of water content, particularly in the nucleus pulposus, is associated with disc degeneration, and with a decrease in disc height and abnormal loading of other spinal structures.
U.S. Pat. No. 8,577,469 to Gross, which is assigned to the assignee of the present application and is incorporated herein by reference, describes apparatus for treating an intervertebral disc of a subject. The apparatus includes a first electrode, configured to be inserted into a nucleus pulposus of the disc, and a second electrode, configured to be placed outside of the nucleus pulposus, in a vicinity of the nucleus pulposus. A control unit is configured to drive a current between the first and second electrodes, and to configure the current to electroosmotically drive fluid between inside and outside the nucleus pulposus. Other embodiments are also described
US Patent Application Publication 2005/0277996 to Podhajsky describes a method for reducing intervertebral pressure, including providing an electrode, having proximal and distal ends, and a generator, which is operatively connected to the proximal end of the electrode, and is configured to supply radiofrequency current thereto. The method also includes inserting at least a portion of the distal end of the electrode into the nucleus pulposus of an intervertebral disc and activating the generator to heat the nucleus pulposus. The electrode may be inserted into the intervertebral disc through its first lateral side and/or its second lateral side, and may be substantially parallel to the major or minor axis of the nucleus pulposus.